To allow an individual to access a protected area, transponder-supported security systems are increasingly used. As an example, a transponder suitable for such a security system is described in EP 0 301 127 B1. An obvious example of such a system is the provision of access to a locked vehicle. The right to unlock the vehicle is to be granted only to authorized individuals. The authorized individual, in this case, carries a transponder, while a control unit is located within the vehicle, which is prompted to transmit an interrogation signal when, for example, the door handle is touched. This interrogation signal causes the transponder to transmit an identification code group, which is received and analyzed by the control unit. When the identification code group matches the identification code group stored in the control unit, the vehicle is unlocked and the individual can now open the unlocked door and so access the inside of the vehicle.
This system proves problematic in practice, as far as the security aspect is concerned. A weak spot, in particular, is the interrogation signal, which is transmitted by the control unit and causes the transponder to re-transmit its identification code group. This interrogation signal is only transmitted at very low power, so that it can be received by the transponder only when this is very close to the aerial that transmits the signal. As soon as a distance of about 2 m is exceeded, the transponder is no longer able to receive the interrogation signal, and the identification code group will not be transmitted. There is, however, a possibility to increase the required restricted distance between the aerial of the control unit and the transponder by a considerable amount, in that a reception coil, together with a connected transmission unit, is located in the proximity of the aerial by a person. This person triggers the transmission of the interrogation signal, in the example considered by touching the door handle of the vehicle, which is then routed onwards by the transmission unit. A second person, in proximity to the authorized individual, carries a receiver that receives the interrogation signal and re-transmits it by means of a transmission aerial. The transponder, carried by the authorized individual, reacts to this interrogation signal as if the authorized individual were in close proximity to the locked vehicle. It therefore transmits the identification code group, which is received and retransmitted by a transceiver carried by the second person. The transceiver carried by the first person receives the identification code group and transmits it, so that it can be received by the aerial of the control unit within the vehicle. Since the identification code group originates from the transponder of the authorized person, this process causes the unlocking of the vehicle door lock, thus enabling the non-authorized person to open the door. This fraudulent procedure is also known as “relay attack”.